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An international team of investigators led by Malaysia released an interim report on Sunday on their inquiry into the disappearance of Malaysia Airlines' Flight MH370 one year after it went missing.

The Boeing 777-200ER disappeared on a flight from Kuala Lumpur to Beijing with 227 passengers and 12 crew on board.

Following are some main points from the team, which included investigators from the United States, Britain, China, France and Australia.

- MH370 was spotted making a turn-back by Malaysian primary radars operated by both the military and civil aviation authorities. Thai radars also spotted MH370, but Bangkok's air traffic controllers "did not pay much attention" as the flight did not fall under Thailand's flight information region.

- Indonesian air traffic control radar in Medan, in the north of Sumatra island, did not pick up MH370 "for unknown reasons" despite having a range of 240 nautical miles. The Indonesian military picked up MH370's radar plot as it was heading towards its last confirmed position, at 35,000 feet about 90 miles off the east coast of Malaysia, heading towards Vietnam, but "no other information was made available".

- Investigations revealed nothing suspicious in the financial, medical or personal histories of pilots or crew, or in behavior before the flight.

- The aircraft's transponder, which was switched off just before the aircraft made the turn-back, was "operating satisfactorily" until it was lost on the air-traffic control screen.

- There was no message from the aircraft reporting a system failure.

- The investigation team found that a battery powering MH370's flight data recorder's underwater locator beacon, which will send a signal if a crash occurs in the water, had expired in December 2012 and had not been replaced. This was because the engineering department's computer system was not properly updated.

- This was not noted until after MH370 disappeared, and Malaysia Airlines (MAS) engineers carried out a fleet-wide inspection of underwater locator beacons to ensure that the records for all of its aircraft were updated.

- The failure of the beacon, however, does not affect the recorder itself, and the investigation team did not point out any problems with the black boxes.

- The investigators confirmed the aircraft's right wing-tip was damaged during an accident while taxiing in Shanghai in 2012. It was repaired by a Boeing team there.

- Data from the aircraft's communications addressing and reporting system (ACARS) revealed no unusual engine behavior on take-off.

- Weather reports showed no adverse weather at last point of contact.

- The report included transcripts of increasingly worried exchanges between air-traffic control centers and the MAS ground operations center about the missing plane.

- A final report will not be issued until the investigation has been completed. Until then, the investigation team has to release an interim statement on each anniversary of the disappearance, detailing progress and any safety issues.